Recently, medical endoscopes, such as nasal endoscopes, including insertion sections with reduced tip diameters have been desired, for example, for improved patient comfort. Accordingly, compact endoscopic image-acquisition devices (e.g., CCD and CMOS sensors) have been developed, and the pixel pitch thereof has been decreasing year by year. With the decreasing pixel pitch, assembly tolerances, such as those of the distance between lenses and the distance between image-acquisition devices and objective lenses, have been becoming smaller, and assembly errors of several micrometers can be problematic. In particular, the accuracy of positioning the objective lens relative to the image-acquisition device greatly affects the depth of field of the endoscope, and the positioning of the objective lens relative to the image-acquisition device often requires an accuracy of 1 μm or less.
Conventional endoscopic image-acquisition apparatuses employ a structure in which an objective lens unit frame holding an objective lens and an image-acquisition-device holding frame holding an image-acquisition device are fitted together (see, for example, Patent Literature PTL 1). The objective lens in this structure is focused on the image-acquisition device by adjusting the positions of the objective lens unit frame and the image-acquisition-device holding frame relative to each other before the fitted portions of the two frames are bonded together with a thermosetting resin.
The components of the above endoscopic image-acquisition apparatus and the assembly jig used to fix the objective lens unit frame and the image-acquisition-device holding frame expand thermally when the thermosetting resin is cured by heating in a drying oven. This thermal expansion may cause the positions of the objective lens unit frame and the image-acquisition-device holding frame to deviate from their desired positions.